This invention relates to spark gap devices and particularly the improvements in gas tube surge arresters of the type primarily used for protecting telephone lines and other equipment connected thereto from overvoltage conditions.
Many telephone line protectors, such as station protectors and central office equipment protectors, embody gas tube surge arresters. When a high voltage surge from lightning or a power line is applied to the protected telephone line, a voltage appears across the electrodes of the gas tube causing the gas in the tube to become ionized so that the tube conducts to ground. Sometimes, however, a gas tube fails as a result of leakage of the inert gas in the tube to atmosphere. This can be the result of damage to a tube from mishandling, improper construction, or the like. In any event, when a gas tube fails in this manner it is no longer a suitable line protective device because its breakdown voltage is excessive due to its wide electrode gap, which is then an air gap. Consequently, it has been a common practice to back-up or supplement gas tube arresters with carbon air gap arresters in parallel thereto. Then if the gas tube fails from loss of gas, the telephone line will be protected through the carbon air gap arrester.
However, where a gas tube surge arrester and a carbon air gap surge arrester are both used the cost of line protection increases substantially. Moreover, in most instances the gas tube does function properly and the carbon air gap arrester is really superfluous. Where there is an extremely fast rise in the surge rate on the line, it usually happens that the carbon arrester discharges first; the gas tube may not even fire at all. If there are frequent instances of a rapidly rising surge rate on the line, the life of the gas tube/carbon arrester combination will in effect be dependent upon the life carbon arrester unit, which is much less than that of the gas tube arrester. Thus, in attempting to utilize the dual protection of a gas tube arrester and a carbon air gap arrester the carbon arrester may actually be the primary functioning protector until it shorts out, following which the gas tube arrester serves no useful purpose unless the carbon arrester is removed. Therefore, combination gas tube/carbon arrester units may tend to defeat the purpose of gas tube protection.
The relationship between breakdown or ionization voltage, electrode spacing and gas pressure is known. According to Paschen's law, for a given cathode surface material and type of gas, the breakdown voltage is a function only of the mathematical product of the gas pressure p and the interelectrode spacing d and not upon these two parameters separately. For electrodes of a given area the volume of gas contained between them is proportional to the interelectrode spacing d. Since the concentration of gas molecules is proportional to the pressure, the value pd is proportional to the number of molecules between the electrodes. Thus, from Paschen's law the ionization voltage depends only upon the total number of molecules of gas between the electrodes for any particular cathode material and gas.
Again, considering a particular gas and particular pair of electrodes, a curve can be plotted of ionization or breakdown voltage as a function of pd. Such a curve will show that very high ionization voltages are required for very high values of pd and for very low values of pd as well. At the low point on the curve there will be a value of pd at which the ionization voltage will be a minimum. The reason for the presence of this minimum pd point in the curve is apparent. Assume, for example, that the interelectrode spacing d is fixed and the pressure p may be varied. At low gas pressures there are only a small number of gas molecules present. The mean free path between the gas molecules is relatively large and so the number of electron collisions that cause gas ionization is relatively small. Consequently, in order to increase the energy of the electrons to a high enough level to produce ionization, a sufficiently high voltage must be applied. On the other hand, where the gas pressure is high the number of electron collisions is quite large and the energy gained by each electron per mean free path is small unless the applied voltage is high. For ionization to take place, the energy per mean free path must exceed a certain minimum amount, namely the ionization potential of the gas, and so a high potential will be necessary. Between the extremes of high pressure will be a gas pressure for a minimum ionization potential.